Is it possible to filter countries in the eBay searches

That's not how it goes. Any user can post a higher bid than your current one, there's only a minimun (the bid increment). If your maximum bid is still higher than the new bid, ebay will add the bid increment to the last bid and count it as your new bid, so you're still going to be the highest bidder, as long as no-one surpasses your maximum bid.

Now to why you shouldn't enter 99,999$ as your maximum bid... If your maximum bid is ridiculously high, you'll be sure to win the item, but you may end up paying a lot more than the original value of the product.
In your example, as long as someone is interested enough in the T-shirt to bid up to $100 for it, if your maximum bet was 99,999$ you'd find yourself paying $100.25 for a T-shirt that in other circumstances you'd only have paid $15 for.

eBay cannot remove the limit and leave it open ended. That idea is unheard of. Some manufacturers offer lifetime warranties on their products, but otherwise all retailers have limits on when an item can be returned and a sale can be reversed. You believe the 45 day limit is silly because you waited 50 days. If it was a 50 day limit, you would say it was silly if you had waited 60 days. There has to be an arbitrary amount of time, and it was decided that 45 days was enough to determine that a product has not arrived/been shipped/had an issue occur in shipping... and start the ball rolling on establishing a solution with the seller. The fact of the matter is that you should have brought this to the attention of eBay much sooner. You cannot avoid your responsibility in this, regardless of how you phrase it.

I agree that 30 days for shipping is reasonable. At which time, you should have been in contact with eBay, not waiting an additional 20 days.